Encapsulant with scatterer to tailor spatial emission pattern and color uniformity in light emitting diodes

ABSTRACT

A light emitting device having an encapsulant with scattering features to tailor the spatial emission pattern and color temperature uniformity of the output profile. The encapsulant is formed with materials having light scattering properties. The concentration of these light scatterers is varied spatially within the encapsulant and/or on the surface of the encapsulant. The regions having a high density of scatterers are arranged in the encapsulant to interact with light entering the encapsulant over a desired range of source emission angles. By increasing the probability that light from a particular range of emission angles will experience at least one scattering event, both the intensity and color temperature profiles of the output light beam can be tuned.

This application is a continuation of U.S. application Ser. No.11/818,818, filed on 14 Jun. 2007.

This invention was made with Government support under Contract No. USAF05-2-5507. The Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to light emitting devices and, more particularly,to white light emitting diodes and multi-colored light emitting deviceassemblies with a tuned spatial emission pattern and color temperatureprofile.

2. Description of the Related Art

Light emitting diodes (LED or LEDs) are solid state devices that convertelectric energy to light, and generally comprise one or more activelayers of semiconductor material sandwiched between oppositely dopedlayers. Typically, wire bonds are used to apply a bias across the dopedlayers, injecting holes and electrons into the active layer where theyrecombine to generate light. Light is emitted from the active layer andfrom all surfaces of the LED. A typical high efficiency LED comprises anLED chip mounted to an LED package and encapsulated by a transparentmedium. The efficient extraction of light from LEDs is a major concernin the fabrication of high efficiency LEDs.

LEDs can be fabricated to emit light in various colors. However,conventional LEDs cannot generate white light from their active layers.Light from a blue emitting LED has been converted to white light bysurrounding the LED with a yellow phosphor, polymer or dye, with atypical phosphor being cerium-doped yttrium aluminum garnet (Ce:YAG).[See Nichia Corp. white LED, Part No. NSPW300BS, NSPW312BS, etc.; Seealso U.S. Pat. No. 5,959,316 to Lowrey, “Multiple Encapsulation ofPhosphor-LED Devices”]. The surrounding phosphor material “downconverts”the energy of some of the LED's blue light which increases thewavelength of the light, changing its color to yellow. Some of the bluelight passes through the phosphor without being changed while a portionof the light is downconverted to yellow. The LED emits both blue andyellow light, which combine to provide a white light. In anotherapproach light from a violet or ultraviolet emitting LED has beenconverted to white light by surrounding the LED with multicolorphosphors or dyes.

It is noted that throughout the application reference is made to twodifferent angles of interest. The first is the viewing angle which isshown as exemplary θ_(v) in FIG. 1 a. The viewing angle is measured fromthe optic axis which in this case runs through the center of thehemispherical encapsulant and is perpendicular to the flat edge of theencapsulant. A viewing angle of zero degrees (0°) indicates that theoutput from the encapsulant is being viewed (or measured) from a pointoutside the encapsulant that is directly opposite the source, i.e.,head-on. The viewing angle increases as the device is tilted withrespect to the viewer. A viewing angle of ninety degrees (90°) indicatesthat the output is being measured from an angle that is perpendicular tothe optic axis and even with the flat edge of the encapsulant, i.e.,directly from the side.

The second angle that is referenced is the emission angle which is shownas θ_(e) in FIG. 1 a. The emission angle shares the same optic axis withthe viewing angle. It measures the angle from the optic axis at which alight ray initially propagates in the encapsulant after it is emittedfrom the source. A light ray that initially propagates from the sourcealong the optic axis (e.g., ray R₁) has an emission angle of 0°. Asshown ray θ_(e) is approximately forty degrees (40°). The emission angleincreases as the direction of initial propagation deviates from theoptic axis. An important difference between the two angles is that theoutput profile at a given viewing angle is affected by scattering eventsinside the encapsulant, whereas the emission angle describes thedirection of the light as it is initially emitted from the source beforeit can interact with materials within the encapsulant.

Various coating processes of LEDs have been considered, including spincoating, spray coating, electrostatic deposition (ESD), andelectrophoretic deposition (EPD). Processes such as spin coating orspray coating typically utilize a binder material during the phosphordeposition, while other processes require the addition of a binderimmediately following their deposition to stabilize the phosphorparticles/powder.

A common type of LED packaging where a phosphor is introduced over anLED is known as a “glob-in-a-cup” method. An LED chip resides at thebottom of a cup-like recession, and a phosphor containing material (e.g.phosphor particles distributed in an encapsulant such as silicone orepoxy) is injected into and fills the cup, surrounding and encapsulatingthe LED. The encapsulant material is then cured to harden it around theLED. This packaging, however, can result in an LED package havingsignificant variation of the color temperature of emitted light atdifferent viewing angles with respect to the package. This colorvariation can be caused by a number of factors, including the differentpath lengths that light can travel through the conversion material. Thisproblem can be made worse in packages where the phosphor containingmatrix material extends above the “rim” of the cup in which the LEDresides, resulting in a predominance of converted light emitted sidewaysinto high viewing angles (e.g., at 90 degrees from the optic axis). Theresult is that the white light emitted by the LED package becomesnon-uniform and can have bands or patches of light having differentcolors or intensities.

Another method for packaging or coating LEDs comprises direct couplingof phosphor particles onto the surfaces of the LED using methods such aselectrophoretic deposition. This process uses electrostatic charge toattract phosphor particles to the surface of the LED chip that ischarged. This method can result in improvement of the color uniformityas a function of viewing angle with one reason for this improvementbeing the source of the converted light and unconverted light being atclose to the same point in space. For example, a blue emitting LEDcovered by a yellow converting material can provide a substantiallyuniform white light source because the converting material and LED areclose to the same point in space. This method can presentinconsistencies due to difficulties in controlling electrostatic chargesacross many LEDs in a mass production environment.

A known approach to addressing these inconsistencies to improve thespatial color temperature uniformity of the emitted light is torandomize the path of outgoing light rays using light scatteringparticles. FIGS. 1 a and 1 b illustrate a light emitting device 100employing this approach. FIG. 1 a represents a cross-section of theknown device taken along section line 1 a (shown in FIG. 1 b). A lightsource 102 is disposed on a substrate 104. A layer of downconvertingmaterial 106 covers the light source 102. A reflector 108 is disposedaround the light source 102 on the substrate 104 such that the lightsource 102 is housed in a cavity defined by the reflector 108 and thesubstrate 104. A hemispherical encapsulant 110 is disposed over thelight source 102. The encapsulant 110 may be mounted over the lightsource 102 using an epoxy adhesive, for example, although other mountingmethods may also be used. Light scattering particles 112 are disposedthroughout the encapsulant 110.

Light rays R1-R4 model the paths of exemplary photons that are emittedfrom the source 102. As shown, R1 is emitted and passes through a length(l₁) of the downconverting material 106 where there is a probabilitythat the light experiences a wavelength conversion. It is noted that theprobability that a photon will be downconverted (i.e., absorbed andre-emitted) increases with the distance that the photon travels throughthe downconverting material 106. Thus, R2 which travels a greaterdistance (l₂) through the downconverting material 106 has a greaterchance of being downconverted. It follows that, depending on the shapeof the downconverting layer, the percentage of light that experiences adownconversion upon passing through the downconverting layer 106 is afunction of the angle of emission from the source 102. Without lightscattering particles, the emission spectrum would exhibit a pronouncedpattern, producing a light spot with variances in color temperature andintensity often noticeable to the human eye. Such non-uniformities canrender a light emitting device undesirable for certain applications.

After passing through the downconverting material 106, the light entersthe encapsulant 110. The light scattering particles 112 distributedthroughout the encapsulant 110 are designed to redirect the individualphotons before they are emitted to randomize the point where the photonsexit the encapsulant 110. This has the effect of improving spatial colortemperature uniformity. For example, R1 collides with a light scatteringparticle 112, changes direction, and is emitted as shown. R1 exits theencapsulant 110 at a different point than it would have if no scatteringparticles were present. R3 experiences multiple scattering events. R2and R4 pass through the encapsulant unimpeded. Thus, the lightscattering particles randomize (to a certain degree) the point at whichemitted photons exit the encapsulant 110 by disassociating the photonsfrom their initial emission angle.

SUMMARY OF THE INVENTION

One embodiment of a light emitting device according to the presentinvention comprises at least one light emitter. An optical element isarranged above the emitter such that light that is emitted from theemitter passes through the optical element. The optical element haslight scattering particles arranged within it to have a density thatvaries spatially in relation to the emission angle of light propagatingthrough the optical element.

One embodiment of an optical element according to the present inventioncomprises a first material defining the shape of the optical element,with the first material having a first refractive index. A secondmaterial having a particulate characteristic is dispersed within thefirst material such that the second material has a non-uniform densitythroughout the first material, the second material having a secondrefractive index.

One method of fabricating an optical element according to the presentinvention comprises the following steps. A mold for shaping the opticalelement is provided. An amount of a first material having particularlight scattering properties is introduced into the mold. Additionalmaterials having particular light scattering properties are introducedinto the mold in a sequence such that the optical element comprisesdistinct regions, each of the regions having particular light scatteringproperties.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a cross-sectional representation of a known light emittingdevice.

FIG. 1 b is a top-side plan view of a known light emitting device.

FIG. 2 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 3 is a graph modeling an exemplary correlated color temperatureoutput profile from a light emitting device with a high density regionaccording to the present invention and a similar device without a highdensity region.

FIG. 4 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 5 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 6 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 7 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 8 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 9 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 10 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 11 is a cross-sectional representation of an embodiment of anencapsulant according to the present invention.

FIG. 12 is a cross-sectional representation of an embodiment of a lightemitting device according to the present invention.

FIG. 13 is a cross-sectional representation of an embodiment of a lightemitting device according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an improved light emitting device andmethods for fabricating the device wherein the emission intensity andcolor temperature profiles can be tuned using materials that have lightscattering properties by arranging the materials in variousconfigurations in an encapsulant around an emitter. The new devices andmethod work particularly well with solid state light sources, such aslight emitting diodes (LEDs). Similarly as in other LED devices, a biasvoltage is applied across the device and light is emitted as a result ofradiative recombination in the active region of the device. It is oftendesirable to engineer the output of an LED, sometimes referred to as thelight spot. Some applications require a light spot with a high degree ofcolor temperature uniformity and a wide emission profile.

Two attributes of the light output profile that can be manipulated usingthe present invention are the color temperature and the intensityprofile as a function of the viewing angle. Other attributes may bemanipulated as well. An encapsulant element is disposed above the lightsource such that substantially all of the light emitted from the sourcehas to pass through it. The encapsulant can also be disposed such thatthe encapsulant and the light source are mounted to a common surface.The encapsulant may comprise any structure that is disposed above thesource as described above and in one embodiment according to the presentinvention the encapsulant can comprise a lens used alone or incombination with other bonding materials to mount the lens over thesource. The encapsulant can be made of silicone, epoxy, glass, plasticor other materials and may perform functions such as beam shaping,collimating, and focusing, etc. The encapsulant may be formed in placeover the source, or it may be fabricated separately and thensubsequently attached to the light source by an adhesive epoxy, forexample. By varying the light scattering properties spatially within theencapsulant, a percentage of the light emitted from a source over arange of emission angles can be redirected to create a desired outputprofile. Emission angles and viewing angles are discussed above inparagraphs [0005] and [0006]. Some exemplary configurations ofencapsulants are discussed in detail below.

Although there are several structures that can be used to scatter lightinside the encapsulant, two light scattering structures that areparticularly well-suited to the present invention are scatteringparticles and surface modifications. By varying the density of the lightscattering particles within the encapsulant to create highlyconcentrated regions of particles, the light from the source can beredirected to achieve a particular output profile.

Another way to redirect light is to modify selected areas of theencapsulant surface. The surface can be modified by several knownmethods such as etching or grinding, for example, as discussed in detailbelow. Light approaching a modified portion of the encapsulant surface(as opposed to an unmodified portion) has a higher probability of beingredirected and exiting the encapsulant at another point. Thus, bymodifying specific regions of the surface the output profile can betailored to specification. Combinations of scattering particles withinthe encapsulant and modifications to the surface of the encapsulant canalso be effective.

It is understood that when an element such as a layer, region orsubstrate is referred to as being “on” another element, it can bedirectly on the other element or intervening elements may also bepresent. Furthermore, relative terms such as “inner”, “outer”, “upper”,“above”, “lower”, “beneath”, and “below”, and similar terms, may be usedherein to describe a relationship of one layer or another region. It isunderstood that these terms are intended to encompass differentorientations of the device in addition to the orientation depicted inthe figures.

Although the terms first, second, etc. may be used herein to describevarious elements, components, regions, layers and/or sections, theseelements, components, regions, layers and/or sections should not belimited by these terms. These terms are only used to distinguish oneelement, component, region, layer or section from another region, layeror section. Thus, a first element, component, region, layer or sectiondiscussed below could be termed a second element, component, region,layer or section without departing from the teachings of the presentinvention.

It is noted that the terms “layer” and “layers” are used interchangeablythroughout the application. A person of ordinary skill in the art willunderstand that a single “layer” of material may actually compriseseveral individual layers of material. Likewise, several “layers” ofmaterial may be considered functionally as a single layer. In otherwords the term “layer” does not denote an homogenous layer of material.A single “layer” may contain various scattering material concentrationsand compositions that are localized in sub-layers. These sub-layers maybe formed in a single formation step or in multiple steps. Unlessspecifically stated otherwise, it is not intended to limit the scope ofthe invention as embodied in the claims by describing an element ascomprising a “layer” or “layers” of material.

Embodiments of the invention are described herein with reference tocross-sectional view illustrations that are schematic illustrations ofidealized embodiments of the invention. As such, variations from theshapes of the illustrations as a result, for example, of manufacturingtechniques and/or tolerances are expected. Embodiments of the inventionshould not be construed as limited to the particular shapes of theregions or particles illustrated herein but are to include deviations inshapes that result, for example, from manufacturing. A regionillustrated or described as rectangular, for example, will typicallyhave rounded or curved features due to normal manufacturing tolerances.Thus, the regions illustrated in the figures are schematic in nature andtheir shapes are not intended to illustrate the precise shape of aregion or particle and are not intended to limit the scope of theinvention.

FIG. 2 shows an embodiment of an encapsulant 200 according to thepresent invention. The encapsulant 200 typically comprises at least twodifferent materials. A medium 202 gives the encapsulant 200 shape. Apreferred shape for the encapsulant 200 is a hemisphere having a curvedsurface and a flat surface. However, many other encapsulant shapes canalso be used such as a flat shape or planoconvex, for example. Themedium 202 comprises thermally or optically curable materials, such astransparent epoxy or silicone, for example. Light scattering particles204 are distributed throughout the medium 202.

Scattering particles 204 can comprise many different materials,including:

silica gel;

zinc oxide (ZnO);

yttrium oxide (Y₂O₃);

titanium dioxide (TiO₂);

barium sulfate (BaSO₄);

alumina (Al₂O₃);

fused silica (SiO₂);

fumed silica (SiO₂);

aluminum nitride;

glass beads;

zirconium dioxide (ZrO₂);

silicon carbide (SiC);

tantalum oxide (TaO₅);

silicon nitride (Si₃N₄);

niobium oxide (Nb₂O₅); or

boron nitride (BN).

TiO₂, Al₂O₃, and silica are preferred materials. Materials other thanthose listed may also be used. These scattering particles 204 shouldhave a high index of refraction relative to the surrounding medium 202,creating a large index of refraction differential between the materials.Because the index differential causes refraction it would also bepossible to use a scattering particle material that has a low index ofrefraction relative to the surrounding medium 202. The diameter of thescattering particles 204 is typically less than a micrometer, althoughlarger particles can be used. The particles 204 create localizednon-uniformities in the medium 202 that force the light to deviate froma straight path.

When the light strikes one or more of the scattering particles 204 theindex of refraction differential between the medium 202 and the particle202 causes the light to refract and travel in a different direction. Alarge index of refraction differential yields a more drastic directionchange for an incident photon. For this reason, materials with a highindex of refraction work well in mediums such as silicone or epoxy.Another consideration when choosing a light scattering material is theoptical absorbance of the material. Large particles back-scatter more ofthe light inside the package before it can escape the encapsulant 200,decreasing the total luminous output of the device. Thus, preferredscattering particle materials have a high index of refraction relativeto the medium (e.g., TiO₂ in epoxy) and a particle size comparable tothe wavelength of the light propagating through the encapsulant 200(e.g., 1 μm particles for the visible spectrum). This ensures maximumforward or sideways scattering effect while minimizing light loss due toback-scattering.

A single photon may experience several scattering events before it isemitted from the encapsulant into the ambient. When a photon passes intoa region with a high density of scattering particles, it may berefracted many times in many directions, making it less probable thatthe photon will exit the encapsulant from a region with a highconcentration of scattering particles. By varying the concentration oflight scattering particles throughout the medium 202, the colortemperature and intensity profiles of the output light can be tailored.

Various concentration levels of scattering particles can be used asdictated by the application for which the device is designed. Using TiO₂scattering particles, for example, a high density region can includeapproximately 0.1% scattering particles by volume while the surroundingmedium can comprise 0.02% scattering particles by volume. Thus, the highdensity region in this example has five times as many scatteringparticles per unit volume as the surrounding medium. The exemplarydensity ratio is 5:1 (high density region:low density region). Otherdensities and density ratios can be used; however, the loss due toabsorption increases with the density of the scattering particles. Thus,in the example above the density of the TiO₂ scattering particles in thesurrounding medium should not exceed 0.05% in order to maintain anacceptable loss figure. Densities and density ratios can vary accordingto the materials selected for the scattering particles and thesurrounding medium.

High density regions can be specifically arranged within the encapsulantto achieve various output profiles by affecting the probability thatlight emitted from the source at a specific angle will exit theencapsulant 200 at a given point. More specifically as discussed above,because the color temperature of the light is a function of the viewingangle, the angular color temperature profile can be controlled. Andbecause it is less likely that light, regardless of color, will passthrough a high density region and exit the encapsulant 200, the angularintensity profile can also be tuned. Other factors may also influencethe disposition of high density regions throughout the encapsulant 200.Encapsulant 200 can be arranged to cooperate with a light emittingdevice similar to the device shown in FIG. 1 a.

Referring again to FIG. 2, region 206 has a high concentration ofscattering particles 204 relative to the adjacent region 208. Region 206represents a three-dimensional (3-D) space, occupying a volume at thetip of the substantially hemispherical encapsulant 200. Light rays 208are shown emanating from a source (not shown) positioned at a distancebeneath the encapsulant 200.

For ease of reference, light entering the encapsulant 200 at emissionangles having an absolute value less than approximately 30° is referredto as low angle light. Light having an emission angle with an absolutevalue greater than approximately 30° and less than approximately 60° istermed mid-range angle light. Light with an emission angle having anabsolute value of greater than approximately 60° is referred to as highangle light. The ranges given are only meant to convey a general senseof the emission angle of incident light and should not be construed tolimit the light associated with one of the descriptive terms to a strictrange of emission angles.

The light rays 208 enter at the flat surface of the encapsulant 200 asshown. In this particular embodiment, low angle light will likelycollide with the high density region 206. A higher percentage of the lowangle light that is incident on region 206 will experience scatteringevents than will light that only passes through the adjacent region 208.A reduced percentage of the light incident on region 206 will passdirectly through the region 206. Using this particular geometry, thelight that is emitted from region 206 will exhibit better colortemperature uniformity owing to an increased number of scattering eventsand a reduced intensity due to light that is redirected away from theregion 206, exiting the encapsulant from the adjacent region 208.

In this embodiment, the volume of the high density tip region 206 can bedetermined according to the viewing angle range of the output profilethat is to be manipulated. A larger volume of high density material willaffect the output profile over a broader range of viewing angles. Forexample, if the design requires an altered output profile over theviewing angle range of −45° to 45°, a specific volume of high densitymaterial is needed to fill the tip region 206. Because the geometry isrelatively simple in this embodiment the following simple equation canbe used to find the necessary volume of high density material, where Ris the radius of the substantially hemispherical encapsulant and θ isthe emission angle:

$V = {\pi \cdot R^{3} \cdot \lbrack {{\cos \; \theta} - \frac{\cos^{3}\theta}{3} - \frac{2}{3}} \rbrack}$

In this embodiment, the high density tip region 206 causes a noticeabledecrease in both the output intensity and correlated color temperature(CCT) over the range of viewing angles where the view is obscured by thetip region 206. This has the effect of flattening out the output profilegraph over the specified angle range as shown in FIG. 3. FIG. 3 is onlymeant to provide an example of a typical output profile using the tipregion embodiment. The graph does not reflect actual experimentalresults.

FIG. 4 shows a cross-sectional representation of another embodiment ofan encapsulant 400 according to the present invention. The encapsulant400 can be formed similarly as discussed above, using the same ordifferent materials. Here, region 402 has a high concentration ofscattering particles 404 relative to the adjacent region 406 and isdisposed near the flat surface of the encapsulant 200 that is closest tothe light source (not shown). FIG. 4 shows the high density region 402having wedge-shaped features. In 3-D, the region 402 resembles atruncated inverse cone structure.

One result of the configuration described in FIG. 4 is that light can beredirected away from the high angles back toward the center of theencapsulant 400. The high density region 406 has the effect ofredistributing some of the intensity that would normally be measured athigh viewing angles to the lower viewing angles. This embodiment of theencapsulant 400 appears brighter when viewed at lower viewing angles(e.g., when viewed head on). Thus, the high density region 402 can beused to shape the intensity profile of the beam. The color temperatureuniformity at high viewing angles is already good, so the high densityregion 402 has little effect on the color temperature profile at highviewing angles.

The wedge-shaped features 402 are disposed to define a space where lowangle light can pass into the low density region 406 without firstinteracting with the high density region 402. The distance between thevertices of the wedge-shaped features 402 may be adjusted to increase ordecrease the size of the space that the light passes through to reachthe low density region 406.

FIG. 5 shows a cross-sectional representation of another embodiment ofan encapsulant 500 according to the present invention. Region 502 has ahigher concentration of light scattering particles 504 than the adjacentregion 506. In 3-D the high density region 502 is substantiallytoroidal. Thus the region 502 forms a ring around the perimeter of theencapsulant 500 with a hole in the middle. In this embodiment, lighthaving emission angles in the higher or lower range passes through theencapsulant without interacting with the high density region 502. Lightwith a mid-range emission angle (e.g., ″>40° or θ<50° will be incidenton the high density region 502. Thus, the output profile is affectedmore drastically at the mid-range viewing angles. The width of theregion 502 and the size of the hole can be chosen such that lightemitted from a specific range of intermediate angles interacts with thehigh density region 502.

FIG. 6 shows a cross-sectional representation of an encapsulant 600according to the present invention. The encapsulant 600 incorporatesmore than one region having a higher concentration of scatteringparticles 602 than the adjacent region 604. Tip region 606 and baseregion 608 are both high density regions. This particular embodimentallows light from mid-range emission angles to pass through theencapsulant 600 with less probability of interacting with the highdensity regions 606, 608. The high density region 608 functions toredirect light back towards the optic axis, shaping the intensityprofile of the beam and redirecting light toward the high density region606. The high density region 606 functions to improve the coloruniformity and the intensity distribution of light at low viewingangles. Although this embodiment shows a particular combination ofregion geometries, many different combinations are possible depending onthe desired output profile. The combinations are only meant to beexemplary. Thus, the invention should not be limited by these examples.

FIG. 7 shows a cross-sectional representation of an encapsulant 700according to the present invention. The encapsulant 700 featuresmultiple high density regions, each of those regions having a differentconcentration of light scattering particles 702. Tip region 704 has thehighest density of light scattering particles 702; the base region 706is less dense than the tip region 704 but more dense than region 708.Densities can be chosen to affect light emitted from discrete ranges ofemission angles differently. A denser region will result in an outputprofile at an associated range of viewing angles that is less intensewith improved color temperature uniformity over some viewing angleranges.

FIG. 8 shows a cross-sectional representation of an embodiment of anencapsulant 800 according to the present invention. The encapsulant 800features a range of scattering particle densities in the gradated tipregion 802. The scattering particles 804 are disposed in a gradient withthe least dense sub-region 806 closest to the source and the densestsub-region 808 at the tip. A sub-region 810 with an intermediate densityis interposed between. The sub-regions 806, 808, 810 are shown asdiscrete layers with homogenous scattering particle densities within.However, the gradated region may be a continuum with a smooth transitionfrom low to high density. Also, the densest sub-region may be disposedclosest to the source (not shown). The gradated tip region 802 affectsthe output profile in a more continuous and smooth fashion over thedesired range of viewing angles, eliminating noticeable intensity andcolor temperature variations.

One method for fabricating a device with gradated scattering particleregions involves a sequential molding process. In the case of anembodiment having a hemispherical encapsulant such as encapsulant 800, ahemispherical mold can be used to form the encapsulant 800. An amount ofa first material having a particular concentration of light scatteringparticles is introduced into the mold. The first material, which in thisembodiment will constitute the tip region, can be allowed to harden orset before adding the next layer, or the process can continue withouthardening. Then, an amount of a second material having a differentconcentration of light scattering particles is introduced into the moldon top of the first material. The second material may be allowed to setbefore adding additional layers, but the process can continue beforehardening of the previously disposed layers. Additional layers havingvarious thicknesses and concentrations of light scattering particles canbe subsequently introduced into the mold. Thus, a sequenced moldingprocess can be used to fabricate an encapsulant such as the one shown inFIG. 8. Many different mold shapes and material sequences may be used tofabricate a desired encapsulant.

FIG. 9 shows a cross-sectional representation of an embodiment of anencapsulant 900 according to the present invention. The encapsulant 900features a modified surface 902 and scattering particles 904. Similarlyas with other scattering materials, the modified surface 902 scattersthe photons of light, preventing them from exiting the encapsulant 900from the same angle at which they were emitted from the source (notshown). This has the effect of randomizing the portion of the emittedlight that is incident on the modified surface 902. Light that strikesthe modified surface 902 has a higher probability of being emitted at analtered angle or being redirected back inside the encapsulant 900. Thus,the intensity and the color temperature profile can be tuned bymodifying a particular portion of the surface.

In this embodiment, the uniformly dense scattering particles 904 have ageneral scattering effect, whereas the modified surface has aconcentrated effect on the output profile over a specific range ofviewing angles. Here, the modified surface 902 is disposed at the tip ofthe encapsulant 900. Light that is emitted at low angles is more likelyto strike the modified surface 902. Because the color temperature is afunction of the emission angle, a specific range of viewing angles canbe targeted for improved color temperature uniformity.

There are several different known methods for modifying a surface.Portions of the surface may be etched, cut, or ground, for example.Other methods of roughening a surface may also be used. A surface may berandomly modified, or it can be specifically textured to provide a moreordered modification. Known methods of texturing can be used to providemany different specific geometric structures on a modified surface suchas truncated pyramids, for example. The degree to which the surface willscatter incident light depends on the roughness of the surface.Roughness can be measured as the average distance from peak to valley ofthe surface contour. As surface roughness increases, the percentage ofscattered light also increases. If, for example, the surface is beingroughened using a chemical etch (e.g., an HF-based etchant) theroughness of the surface can be tuned by varying the etch time. A longeretch time will typically result in a higher degree of surface roughness.In this way, the surface roughness can be controlled to achieve aparticular average level of scattering.

FIG. 10 shows a cross-sectional representation of an embodiment of anencapsulant 1000 according to the present invention. The encapsulant1000 features multiple modified surfaces 1002, 1004. Similarly as withhigh density scattering particle regions, light from various emissionangle ranges can be manipulated to yield a specific output profile. Inthis embodiment light from both low angles and high angles will beredirected internally at a higher percentage than light emitted at anintermediate angle. The light emitted from these surfaces 1002, 1004will also exhibit a more uniform color temperature distribution,although the effect will be more noticeable at lower viewing angleswhere the color temperature non-uniformity is at a maximum. By selectingcertain areas of the encapsulant 1000 surface to modify, the outputintensity profile can be specifically tuned. FIG. 10 is an exemplaryembodiment of a combination of modified surface regions. However, manyother modified surface geometries can also be used to achieve tailoredoutput profiles.

FIG. 11 shows a cross-sectional representation of an embodiment of anencapsulant 1100 according to the present invention. The encapsulantfeatures a combination of a high density scattering particle region 1102and a modified surface 1104. This particular exemplary embodiment wouldaffect the output profile with respect to light that is emitted fromboth low and high angles. The high density region 1102 is arranged tointeract with the light that is emitted from the source at low angles.The modified surface 1104 is disposed to alter the profile of lightemitted from the source at high angles and to redirect high angle lightback into the encapsulant 1100 and towards region 1102. Although bothhigh density regions and modified surfaces have a similar effect onlight that interacts with them, there may be differences in the outputprofiles resulting from the two different structures. The combinationmay provide advantages due to the manner and the different degree towhich the structures interact with the light. Using both kinds ofstructures in a single encapsulant can provide additional design optionsto yield a highly specific output profile. Many variations on thecombination arrangement are possible. For example, a high density regioncan be used to modify the profile of light emitted from mid-range angleswhile a modified surface interacts with the light emitted at low angles.The invention is not limited to any particular combination orarrangement.

FIG. 12 shows a cross-sectional representation of an embodiment of alight emitting device 1200. A light source 1202 such as an LED, forexample, is disposed on a surface with the source's primary emissionsurface covered by a layer of wavelength conversion material 1204. Aring-shaped reflector element 1206 is disposed on the surface andsurrounds the source 1202. The reflector element 1206 may be made of areflective material such as aluminum, for example, or it may have adiffusive or reflective coating on its inner wall that faces the source1202. The reflective element 1206 redirects light that is emitted fromthe source 1202 at very high angles.

An encapsulant 1208 is disposed above the source 1202 such thatsubstantially all of the light that is emitted must pass through theencapsulant 1208 before it escapes into the ambient. Although theencapsulant 1208 can be many shapes, a preferred shape is a hemisphere.Light scattering particles 1210 are distributed throughout the lighttransmitting encapsulant 1208. The encapsulant 1208 may be mounted abovethe source 1202 using a light transmitting filler material 1209. Thefiller material 1209 is preferably high temperature polymer with highlight transmissivity and a refractive index that matches or closelymatches the refractive index of the encapsulant 1208, which may be madefrom glass, quartz, high temperature and transparent plastic, silicone,epoxy resin or a combination of these materials. The encapsulant 1208can be placed on top of and adheres to the filler material 1209. In analternative embodiment, the encapsulant may be formed such that theencapsulant and the light source are mounted to a common surface with nofiller material in between.

In this particular embodiment, the encapsulant comprises a modifiedsurface 1212. The modified surface 1212 is arranged at the tip of theencapsulant 1208, interacting with light emitted from the source 1202 atlow angles (i.e., along the optic axis). The reflector element 1206 alsocomprises a modified surface 1214 that runs along the inner wall, facingtoward the source 1202. The surface 1214 may be modified similarly asdiscussed above with respect to modified surfaces of an encapsulant. Forexample, the surface may be roughened/textured by etching, cutting orgrinding. Other methods of surface modification may also be used.Although not shown in FIG. 12, it is also possible to modify the flatsurface of the encapsulant 1200 that faces the source 1202. The modifiedsurface 1214 randomizes the direction of high angle light before itpasses into the encapsulant 1208 above. This helps to eliminate thecolor temperature pattern caused by wavelength conversion material asdiscussed above. The modified surface 1214 works in addition to thescattering particles 1210 and the modified encapsulant surface 1212. Themodified surface 1214 can be used in combination with any other lightscattering structures to achieve a tailored output profile.

FIG. 13 shows a cross-sectional representation of an embodiment of alight emitting device 1300 according to the present invention. Thedevice 1300 is similar to the device shown in FIG. 12 and has many ofthe same features. The device 1300 comprises several light scatteringelements. An encapsulant 1302 has light scattering particles distributedwithin it, some of which are concentrated in a high density region 1304at the tip of the encapsulant 1302. The device 1300 features modifiedencapsulant surfaces 1306, 1307. A reflector element 1308 also has amodified surface 1310.

The reflector element 1308 is mounted on a surface 1312 along with aplurality of light sources 1314. The light sources 1314 can be the samecolor or different colors, monochromatic or white. In a preferredembodiment, three light sources are mounted to the surface 1312: a redsource, a green source, and a blue source. The light sources 1314 may bemounted in a variety of configurations on the surface 1308. Thescattering elements can be arranged within the device such that lightfrom some or all of the sources 1314 can be manipulated to achieve adesired output profile.

Although the present invention has been described in detail withreference to certain preferred configurations thereof, other versionsare possible. Therefore, the spirit and scope of the invention shouldnot be limited to the versions described above.

1. A light emitting device, comprising: at least one light emitter; andan optical element arranged proximate to said emitter such that lightthat is emitted from said at least one emitter passes through saidoptical element, said optical element comprising light scatteringparticles arranged within said optical element to have a density thatvaries spatially in relation to the emission angle of light propagatingthrough said optical element.
 2. The light emitting device of claim 1,wherein said optical element comprises multiple three-dimensional (3-D)regions containing said light scattering particles, said 3-D regionsarranged within said optical element to tune the intensity profile ofthe light emitted from said at least one light emitter as a function ofthe viewing angle.
 3. The light emitting device of claim 2, wherein saidoptical element is hemispherical and has a bisecting cross-section, saidcross-section showing two wedge-shaped areas extending in oppositedirections along the bottom surface of said optical element such thatsaid wedge-shaped areas are coextensive with the outer surface of saidoptical element, said wedge-shaped areas defining one of said 3-Dregions having a high concentration of light scattering particlesrelative to an adjacent region.
 4. The light emitting device of claim 2,one of said 3-D regions having a high concentration of light scatteringparticles relative to an adjacent region, said 3-0 region disposed in asubstantially toroidal region having an outer radius that is coextensivewith the surface of said optical element and an inner radius at adistance from the center of said optical element.
 5. The light emittingdevice of claim 1, wherein one or more portions of the surface of saidoptical element are modified to scatter emitted light incident on saidsurface.
 6. The light emitting device of claim 1, further comprising alayer of wavelength conversion material surrounding the exposed portionsof said at least one light emitter.
 7. The light emitting device ofclaim 1, further comprising a reflector disposed on said substrate, saidreflector having a substantially toroidal shape with said opticalelement and said at least one emitter arranged in the center, saidreflector comprising inner walls to redirect light from said at leastone emitter toward said optical element.
 8. An optical element,comprising: a first material defining the shape of said optical element,said first material having a first refractive index; and a secondmaterial having a particulate characteristic dispersed within said firstmaterial such that said second material has a non-uniform densitythroughout said first material, said second material having a secondrefractive index.
 9. The optical element of claim 8, wherein saidoptical element comprises a high density region having a higherconcentration of said second material than adjacent regions within saidoptical element.
 10. The optical element of claim 9, wherein saidoptical element has a convex curved surface and a flat surface.
 11. Theoptical element of claim 10, wherein said optical element is positionedto receive light from at least one source, said light incident on saidflat surface.
 12. The optical element of claim 11, wherein said highdensity region is disposed within said optical element such that saidhigh density region interacts with light that is emitted from said atleast one source over a range of low emission angles.
 13. The opticalelement of claim 12, wherein said high density region is disposed in thetip of said optical element.
 14. The optical element of claim 11,wherein said high density region is disposed within said optical elementsuch that said high density region interacts with light that is emittedfrom said at least one source over a range of high emission angles. 15.The optical element of claim 14, wherein said high density region isdisposed along the flat edge of said optical element such that said highdensity region forms an inverse cone within said optical element. 16.The optical element of claim 11, wherein said high density region isdisposed within said optical element such that said high density regioninteracts with light that is emitted from said at least one source overa middle range of emission angles.
 17. The optical element of claim 16,wherein said high density region has a substantially toroidal shape. 18.The optical element of claim 11, wherein said high density region isdisposed within said optical element such that said high density regioninteracts with light that is emitted from said at least one source overa combination of ranges of low angles, mid-range angles and/or highangles.
 19. A method of fabricating an optical element, comprising:providing a mold for shaping said optical element; introducing an amountof a first material having particular light scattering properties intosaid mold; and introducing additional materials having particular lightscattering properties into said mold in a sequence such that saidoptical element comprises distinct regions, each of said regions havingparticular light scattering properties.
 20. The method of claim 19,wherein each of said materials introduced into said mold is allowed toset before introducing the next of said materials in said sequence intosaid mold.
 21. The method of claim 19, wherein said materials includelight scattering particles.
 22. The method of claim 21, wherein saidlight scattering properties of said materials are at least partiallydetermined by the concentration of said light scattering particles ineach of said materials.
 23. The method of claim 21, wherein said opticalelement is substantially hemispherical, and wherein said first materialhas the highest concentration of light scattering particles relative tosaid additional materials, said additional materials disposed to havedecreasing concentrations of light scattering particles with increasingdistance from the tip of said optical element.
 24. The method of claim19, wherein said optical element comprises at least three of saiddistinct regions.